Cameron's heir? PM sees bright future for Boris

David Cameron has offered his strongest hint yet that he views Boris Johnson as his potential successor, telling him he has "a huge amount to offer" once he leaves City Hall

The Conservative leader and prime minister showered praise on the mayor of London in a series of interviews as his party's conference got underway in Birmingham.

"I like having other people in the Conservative party who are popular, who get out there, who talk our message and explain our ideas and values. Boris is fantastic at that. He's one of the politicians people warm to," Cameron told BBC1's The Andrew Marr Show.

"I've said to Boris, once you've done your job as London mayor, don't think your job in politics is over. I think he's got a huge amount to offer and a huge amount to give. I encourage him to do that."

Johnson has had a good year, seeing off a re-election challenge from Labour's Ken Livingstone at the polls and enjoying huge popularity during the London 2012 Olympic and Paralympic Games.

He will receive another dose of adulation from Tory supporters tomorrow evening at a rally in Birmingham called 'Boris Johnson's 2012: re-elected and Olympotastic'.

Johnson has used his political standing to escalate his row with the government over its aviation strategy, warning that further inaction could lead to "economic catastrophe".

Cameron is coping with the assaults by shrugging his shoulders.

"There's no point trying to contain Boris. He's mayor of London, he can speak out if he wants to," the prime minister said in an interview with the Sunday Telegraph.

"I think it's really important we make the right decision about Britain’s future airport capacity, but, as I said, do we need to make a decision in the next six months? No. Is it critical to our growth plan, that we start building a new airport somewhere in the next six months? No, it isn't. I mean, it's an important question for the future.

"So I have to be, and I am, relatively, as you can see, relaxed about having the blond haired mop sounding off from time to time. Every premiership has its backdrop, and I suspect this is just going to be one of mine."

That backdrop is looking increasingly gloomy for Cameron, however. An Opinium poll for the Observer newspaper put his net approval ratings on -21%, compared to +30% for Johnson.

Cameron's public professions that attempting to handle Johnson is futile clashes with reports in today's newspapers that he has been asked to clear his speech to conference, which takes place on Tuesday, with Downing Street.

"There is fear at No 10 that he could upstage the PM," a source told the Sunday Times.

"He has been asked to clear everything, which he has reluctantly agreed to do."